1. Field of the Invention
This invention relates to an optical device for photomagnetic recording used in a recording/reproducing apparatus for recording or reproducing information signals for an optical recording medium, such as a magneto-optical disc. The present invention also relates to an optical device and an optical head recording and/or reproducing apparatus employing this optical device.
2. Description of the Related Art
In an optical disc device for recording or reproducing information signals for a magneto-optical disc as a recording medium, there is recently proposed an optical disc device in which an optical system is provided towards the photomagnetic recording layer of the magneto-optical disc to increase the numerical aperture to achieve high density recording and in which the optical system is unified to a magnetic coil to realize the reduced thickness of the device.
This optical disc device uses an optical device 100 having two lenses, shown in FIG. 1, as an objective lens for the optical head.
In the present optical device 100, one of the two lenses towards a magneto-optical disc 101 is a semispherical lens. This lens towards the magneto-optical disc 101 is termed herein as a forward lens 102 and the other lens is termed herein as a backward lens 103. On a spherically-shaped surface 102a of the forward lens 102 is formed a thin film coil 104.
The optical device 100 is configured so that the light radiated from a light source to fall on the optical device 100 is converged by the backward lens 103 and the forward lens 102 to pass through a center hole (light transmitting hole 104a) of the thin film coil 104 so as to be illuminated on the photomagnetic recording layer of the magneto-optical disc 101.
The optical device 100 is also configured so that the thin film coil 104 generates a magnetic field corresponding to recording signals fed from a predetermined device to apply this magnetic field at a position of the photomagnetic recording layer of the magneto-optical disc 101 illuminated by the light.
Meanwhile, the optical device 100, used in the conventional optical disc device, is configured so that the light converged by the backward lens 103 and the forward lens 102 is caused to pass through the center hole (light transmitting hole 104a) of the thin film coil 104 so as to be illuminated on the photomagnetic recording layer of the magneto-optical disc 101.
With the optical device 100, the diameter of the light transmitting hole 104a of the thin film coil 104 is set so that, if a pre-set amount of eccentricity of light from the center axis due to levelling of the optical axis or the assembling error is produced, the light illuminated on the photomagnetic layer of the magneto-optical disc 101 is not kicked by the thin film coil 104.
That is, with this optical device 100, there are occasion s wherein, due to the assembling error or tilt caused during the operation, the optical axis is tilted by approximately 5 mrad at the maximum, as shown in FIG. 2A. Due to this levelling of the optical axis, there are occasions wherein the center of light converged by the backward lens 103 and the forward lens 102 is offset by approximately 20 μm from the center axis of the optical device 100.
Also, in the present optical device 100, there are occasions wherein, due to the assembling error within the assembling tolerance, the center of the light converged by the backward lens 103 and the forward lens 102 is offset by approximately 10 μm from the center axis of the optical device 100.
If the light converged by the backward lens 103 and the forward lens 102 is offset from the center axis of the optical device 100, and the diameter of the light transmitting hole 104a of the thin film coil 104 is small, there are occasions wherein the light is not transmitted optimally through the light transmitting hole 104a of the thin film coil 104 but is partially kicked by the thin film coil 104 to produce variations in diameter.
If the light illuminated on the photomagnetic recording layer of the optical recording medium 101 undergoes variations in the diameter, optimum playback signals or control signals cannot be produced.
Thus, if, with the optical device 100, used in the conventional optical disc device, the light transmitting hole 104a of the thin film coil 104 is increased in diameter, as shown in FIG. 2B, so that, even if a pre-set amount of eccentricity is produced due to the levelling of the optical axis or the assembling error, there will be no risk of the light illuminated on the photomagnetic recording layer of the magneto-optical disc 101 being kicked by the thin film coil 104.
However, if the light transmitting hole 104a of the thin film coil 104 of the optical device 100 is of a larger diameter, an extremely large current needs to be sent to the thin film coil 104 in order to generate a magnetic field required during recording, thus increasing the power consumption. Moreover, heat evolution in the thin film coil 104 is increased to cause rupture of the thin film coil 104.
In addition, in the optical device 100 used in the above-described conventional optical disc device, the forward lens 102 is formed of a glass member having a thermal conductivity as low as approximately 0.55 to 0.75 W/m·K. With the optical device 100, the thin film coil 104 is directly formed on the circular surface 102a of the forward lens 102 formed by the glass member.
Thus, with the present optical device 100, since the thin film coil 104 is in a thermally insulated state, there are occasions wherein the heat evolved in the thin film coil 104 is stored in the thin film coil 104 itself without being transmitted to the forward lens 102.
Even if the forward lens 102 is formed of quartz glass, the heat evolved in the thin film coil cannot be released sufficiently because the thermal conductivity of the quartz glass is of the order of 1.0 to 2.0 W/m·K.
If the thin film coil 104 is in the thermally insulated state, and the heat generated in the thin film coil 104 is stored in the thin film coil 104 itself, the magnetic field generation efficiency from the thin film coil 104 is lowered especially in case of high modulation frequency, while there is also a risk of firing of the thin film coil 104 itself.
In addition, in the optical device 100 used in the above-described conventional optical disc device, the diameter of the light transmitting hole 104a of the coil 104 is desirably set to a smaller value in order to realize high NA and in order to apply the magnetic field to the photomagnetic recording layer of the magneto-optical disc 101 efficiently with small power consumption. It has, however, been difficult with the above-described optical device 100 to reduce the diameter of the light transmitting hole 104a of the coil 104.
That is, the coil 104 of the optical device 100 has a spirally shaped thin-film coil, the outer periphery of which is connected to an electrode used for supplying the driving current to the coil 104 and the inner periphery of which is connected to a lead-out line provided between the thin film coil and the forward lens 102. This lead-out line is connected to the other electrode to supply the driving current to the coil 104.
Thus, in the present optical device 100, the thickness of the coil 104 is equal to the sum of the thickness of the thin film coil and that of the lead-out line.
There is also proposed an optical device having a coil structure in which a thin film coil is of two layers, the outer periphery of the upper layer coil is connected to an electrode, the outer rim of the lower layer coil is connected to the opposite side electrode and in which the inner periphery of the upper layer coil is connected to the inner periphery of the lower layer coil. In this optical device structure, the thickness of the coil structure is the sum of the thicknesses of the upper layer coil and the lower layer coil, such that the thickness of the coil 104 cannot be reduced beyond a certain limit value.
If, in the optical disc device employing this type of the optical device, the coil 104 of the optical device 100 is of a larger thickness, it is necessary to increase the distance d between the surface of the magneto-optical disc 101 and the forward lens 102 shown in FIG. 3.
That is, if the coil 104 is of a larger thickness, the distance between the coil 104 and the magneto-optical disc 101 (working distance WD) needs to be of a certain fixed value in order for the coil 104 not to collide against the surface of the magneto-optical disc 101. Thus, while the distance d between the surface of the magneto-optical disc 101 and the forward lens 102 is increased if the coil 104 is of a larger thickness, the diameter φ of the light transmitting hole 104a of the thin film coil 104 of the optical device 100 needs to be increased in order to realize a large value of NA if the distance d between the surface of the magneto-optical disc 101 and the forward lens 102 is of a large value.
Specifically, the diameter φ of the light transmitting hole 104a of the thin film coil 104 depends on the distance d between the surface of the magneto-optical disc 101 and the forward lens 102, as shown by the following equation (1):φ≧2(t·tan(θ1)+d·tan(θ2))  (1) where θ1 is an angle of incidence of light transmitted through a cover glass 106 provided on a photomagnetic recording layer 105 of the magneto-optical disc 101 and illuminated on the photomagnetic recording layer 105. This angle of incidence θ1 is represented by the following equation (2):θ1=sin−1(NA/ns)  (2) where ns is the refractive index of the cover glass 106. With the optical device 100,the angle of incidence θ1 is set to approximately 34° to realize high NA.
Meanwhile, 02 is the angle of incidence of light on the cover glass 106. This angle of incidence O2 is expressed by the following equation (3):θ2=sin−1(NA)  (3) 
In the optical device 100, the angle of incidence θ2 is set to approximately 58° to realize high NA.
Meanwhile, t is the thickness of the cover glass 106 which is set to a preset value to realize high NA.
If, in the optical device 100, the diameter φ of the light transmitting hole 104a of the coil 104 is increased, an extremely large current needs to be supplied to the coil 104, thus increasing the power consumption. Moreover, heat evolution in the coil 104 is increased to cause rupture of the coil 104 from time to time.